Tips & Tricks

Interstitials

As described in the YouTube defect calculation tutorial, our recommended workflow for calculating interstitial defects is to first generate the set of possible interstitial sites for your structure using DefectsGenerator (which uses Voronoi tessellation to do this), and then perform Gamma-point-only relaxations (using vasp_gam) for the neutral state of each generated interstitial candidate. The vasp_gam relaxation files can be generated following the steps shown in the defect generation tutorial and setting vasp_gam = True in DefectsSet.write_files().

We can then compare the energies of these trial neutral relaxations, and remove any candidates that either:

• Are very high energy (>1 eV above the lowest energy site), and so are unlikely to form.

• Relax to the same final structure/energy as other interstitial sites (despite different initial positions), and so are unnecessary to calculate. This can happen due to interstitial migration within the relaxation calculation, from an unfavourable higher energy site, to a lower energy one. Typically if the energy from the test neutral vasp_gam relaxations are within a couple meV of eachother, this is the case.

Difficult Structural Relaxations

If defect supercell relaxations do not converge after multiple continuation calculations (i.e. cp-ing CONTCAR to POSCAR and resubmitting the job), this is likely due to small residual forces causing the local optimisation algorithm to struggle to find a solution, an error in the underlying calculation and/or extreme forces.

• If the calculation outputs show that the relaxation is proceeding fine, without any errors, just not converging to completion, then it suggests that the structure relaxation is bouncing around a narrow region of the potential energy surface. Here, the gradient-based geometry optimiser is struggling to converge.

  • Often (but not always) this indicates that the structure may be stuck around a saddle point or shallow local minimum on the potential energy surface (PES), so it’s important to make sure that you have performed structure-searching (PES scanning) with ShakeNBreak (SnB) to avoid this. You may want to try ‘rattling’ the structure to break symmetry in case this is an issue, as detailed in this part of the SnB docs.

  • Alternatively (if you have already performed `SnB` structure-seaerching), convergence of the forces can be aided by:

  • Switching the ionic relaxation algorithm back and forth (i.e. change IBRION to 1 or 3 and back).

  • Reducing the ionic step width (e.g. change POTIM to 0.02 in the INCAR)

  • Switching the electronic minimisation algorithm (e.g. change ALGO to All), if electronic convergence seems to be causing issues.

  • Tightening/reducing the electronic convergence criterion (e.g. change EDIFF to 1e-7)

• If instead the calculation is crashing due to an error and/or extreme forces, a common culprit is the EDWAV error in the output file, which can often be avoided by reducing NCORE and/or KPAR. If this doesn’t fix it, switching the electronic minimisation algorithm (e.g. change ALGO to All) can sometimes help.

  • If some relaxations are still not converging after multiple continuations, you should check the calculation output files to see if this requires fixing. Often this may require changing a specific INCAR setting, and using the updated setting(s) for any other relaxations that are also struggling to converge.

ShakeNBreak

For tips on the ShakeNBreak part of the defect calculation workflow, please refer to the ShakeNBreak documentation.

Have any tips for users from using doped? Please share it with the developers and we’ll add them here!